FT-IR microscopes are used to analyze small samples of material. The microscope has a viewing configuration and a measurement configuration. In both configurations the microscope can be used either in a transmitting mode or a reflecting mode, depending upon the nature of the sample. Typically such a microscope is used in conjunction with an IR spectrophotometer. A microscope of this type generally includes a source of visible radiation and can receive analyzing infrared radiation from a source in the spectrophotometer. A typical microscope includes a sample stage for carrying a sample to be investigated and optical elements for guiding radiation from one or other radiation sources to the sample stage. These elements can include a plane mirror, a toroidal coupling optic and a Cassegrain mirror assembly acting as a condenser. A microscope also includes a Cassegrain mirror assembly which images the sample at a given magnification at an intermediate image plane from where the radiation is directed to an infrared detector. The microscope also includes an optical microscope which enables an image sample on the stage to be viewed optically by means of visible radiation and thereby enables areas of interest to be identified. The microscope can also include a video camera which can be used in conjunction with the optical microscope in order to create an image of the sample for display on display means of a computer which is used to control the microscope.
Modern microscopes of this type have a stage which can be moved under computer control to allow several areas of interest to be identified, their coordinates stored and data collected subsequently automatically on the basis of that stored data. Such microscopes also include a variable aperture which can be computer controlled and is located at the intermediate image plane to mask off a portion of the sample. This combined with an oversized single detector element enables the measurement of the infrared spectrum of selected areas of the sample. By stepping the stage and repeating the measurement, the system can slowly build-up a digital image of the sample pixel-by-pixel. An arrangement of this type is described in EP-A-0731371. Typically such microscopes employ a liquid nitrogen cooled, photoconductive mercury cadmium telluride (MCT) element as the infrared detector. A microscope with a single detector has relatively long measurement times and it can take of the order of 10 hours to acquire a 128×128 pixel image.
In order to reduce measurement times, microscopes have been designed which incorporate large detector arrays rather than single detector elements. One known arrangement used in an integrated array of 64×64 liquid nitrogen cooled photovoltaic MCT detectors, each having an area of 60 square microns. However, these arrangements have turned out to be very expensive partly because of the increased cost of the detector itself and partly due to the fact that the slow read-out of the multiplexed detector necessitates the use of a sophisticated spectrometer technology called step scan.
We have proposed in European Patent Application No 00307372.3 to use a relatively small detector array whose outputs are sufficiently small in number so that they can be processed without the need for complex multiplexing of the outputs. Typically the detector array has 16 detector elements.